Electrical test probes are used to transmit a signal voltage from a circuit under test to an electronic test instrument, such as an oscilloscope, logic analyzer, voltmeter, or ammeter, among other test instruments. One desirable characteristic for a test probe is that the signal response at the test instrument, e.g., the probe output signal, is an accurate (although attenuated) representation of the probed test signal over a range of frequencies of interest. Another desirable characteristic is that the probe does not influence, e.g., “load,” the response of the circuit under test.
Some test probes attempt to provide such characteristics by exhibiting both high resistance and low capacitance. A high probe resistance allows relatively little of the output current to flow through the probe, thereby decreasing any loading effect of the probe on the circuit under test. In various test probes, the frequency response of the probe is dependent upon the capacitance of the probe in parallel with the resistance of the circuit under test. Capacitive reactance varies as a function of frequency, causing the impedance of the probe to fall as the applied frequency increases. As such, the effective bandwidth of prior art probes has thus been limited by probe capacitance, thus making some such probes unsuitable for use in the GHz range.
Some test probes that can be used to measure test signals at relatively higher frequencies, e.g., the GHz range and above, use a relatively low input impedance, e.g., 50 ohms, which can be matched to a coaxial input line. However, circuits under test can be severely loaded by the lower impedance such that additional output drivers may need to be used to drive the lower impedance.
To increase the effective bandwidth of test probes, some prior approaches attempt to compensate for, e.g., reduce, probe tip capacitance by using active electronics in the probe tip, e.g., by using active probes which require a source of electrical power. However, such active probes have drawbacks that include relatively large and easily damaged probe tips as well as the use of additional components as well as a source of electrical power.